Inorganic solid electrolyte-containing composition, sheet for all-solid state secondary battery, and all-solid state secondary battery, and manufacturing methods for sheet for all-solid state secondary battery and all-solid state secondary battery

This application is a Continuation of PCT International Application No. PCT/JP2021/035331 filed on Sep. 27, 2021, which claims priority under 35 U.S.C. § 119 (a) to Japanese Patent Application No. 2020-162143 filed in Japan on Sep. 28, 2020. Each of the above applications is hereby expressly incorporated by reference, in its entirety, into the present application.

The present invention relates to an inorganic solid electrolyte-containing composition, a sheet for an all-solid state secondary battery, and an all-solid state secondary battery, and manufacturing methods for a sheet for an all-solid state secondary battery and an all-solid state secondary battery.

A secondary battery is a storage battery that includes a negative electrode, a positive electrode, and an electrolyte between the negative electrode and the positive electrode and enables charging and discharging by the reciprocal migration of specific metal ions such as lithium ions between both electrodes.

Examples of the representative secondary battery include a secondary battery in which a non-aqueous electrolyte such as an organic electrolytic solution is filled between a negative electrode active material layer and a positive electrode active material layer. This non-aqueous electrolyte secondary battery exhibits relatively high battery performance and thus is used in a wide range of use applications. Such a non-aqueous electrolyte secondary battery is manufactured by various methods. For example, the negative electrode active material layer and the positive electrode active material layer are generally formed of a material containing an active material and a binder. For example, JP2018-200889A proposes a positive electrode slurry for a secondary battery which is a positive electrode slurry for a secondary battery, containing a positive electrode active material, a conductive agent, a binder, and a dispersion medium, where the binder contains “a first polymer that is a fluorine-containing polymer” and “a second polymer that has a polymerization unit having a nitrile group, a polymerization unit having a hydrophilic group, a (meth)acrylic acid ester polymerization unit, and a linear alkylene polymerization unit having 4 or more carbon atoms” at a specific mass ratio.

In the above-described non-aqueous electrolyte secondary battery, the non-aqueous electrolyte which is an organic electrolytic solution generally leakages easily, and a short circuit easily occurs in the inside of the battery due to overcharging or overdischarging. As a result, there is a demand for additional improvement in safety and reliability. Under these circumstances, an all-solid state secondary battery in which an inorganic solid electrolyte is used instead of the organic electrolytic solution has attracted attention. In this all-solid state secondary battery, since all of the negative electrode, the electrolyte, and the positive electrode are solid, safety and reliability which are considered as a problem of the non-aqueous electrolyte secondary battery can be significantly improved. It is also said to be capable of extending the battery life. Furthermore, all-solid state secondary batteries can be provided with a structure in which the electrodes and the electrolyte are directly disposed in series. As a result, it becomes possible to increase the energy density to be high as compared with a secondary battery in which an organic electrolytic solution is used, and thus the application to electric vehicles, large-sized storage batteries, and the like is anticipated.

In such an all-solid state secondary battery, examples of substances that form constitutional layers (a solid electrolyte layer, a negative electrode active material layer, a positive electrode active material layer, and the like) include an inorganic solid electrolyte and an active material. In recent years, this inorganic solid electrolyte, particularly an oxide-based inorganic solid electrolyte or a sulfide-based inorganic solid electrolyte is expected as an electrolyte material having a high ion conductivity comparable to that of the organic electrolytic solution. In consideration of the improvement in productivity, a constitutional layer using such an inorganic solid electrolyte is generally formed of a material (a constitutional layer forming material) containing an inorganic solid electrolyte and a binder. As such a constitutional layer forming material, WO2016/017758A1 discloses, for example, a solid electrolyte composition that contains an inorganic solid electrolyte having a conductivity of an ion of a metal belonging to Group 1 or Group 2 of the periodic table and a binder composed of a polymeric compound in which (i) a linking structure of the main chain is composed of carbon atoms, (ii) a repeating unit represented by a specific formula is provided, and (iii) at least one of a specific group of (a) of functional groups.

A constitutional layer forming material used for manufacturing an all-solid state secondary battery is required to have a characteristic (dispersion stability) of stably maintaining the excellent dispersibility of solid particles immediately after preparation. This is because, in a case of using a constitutional layer forming material having excellent dispersion stability, a uniform constitutional layer having no coating unevenness can be formed, and thus the battery performance (for example, the ion conductivity) of a secondary battery can be improved.

By the way, since a constitutional layer of an all-solid state secondary battery is formed of solid particles (an inorganic solid electrolyte, an active material, a conductive auxiliary agent, and the like), there is a unique problem that the interfacial contact state between solid particles is restricted, and it is difficult to exhibit a firm binding force (adhesion) between the solid particles and to a base material to be laminated and the like.

Therefore, in an all-solid state secondary battery including such a constitutional layer, there is a strong tendency that the battery performance (for example, cycle characteristics) is significantly deteriorated due to repeated charging and discharging.

In addition, a constitutional layer having insufficient binding force causes defects such as chipping, breakage, cracking, or peeling in the manufacturing process of an all-solid state secondary battery, and is easily peeled off from a base material, which results in the deterioration (for example, the increase in resistance and the decrease in ion conductivity) of the batter performance of the all-solid state secondary battery. In particular, in a case of employing a highly productive method of continuously manufacturing a constitutional layer as a sheet, problems of the occurrence of defects of the constitutional layer and the peeling from the base material occur remarkably. These problems are conceived to be one of the factors of the vibration stress, the bending stress, and the like which act on a sheet for an all-solid state secondary battery, for example, in a case of winding the sheet for an all-solid state secondary battery around a winding core or in a case of manufacturing it by a roll-to-roll method having high productivity. Further, a factor thereof is also conceived to be a stress that acts on an interface with a constitutional layer or a base material in a case of cutting (punching out) the sheet for an all-solid state secondary battery as a solid electrolyte layer or an active material layer.

Regarding the solid particles that form the constitutional layer, the relationship among the solid particles such as the inorganic solid electrolyte, the dispersion medium, and the binder in the constitutional layer forming material and the constitutional layer is conceived to be one of the important factors in the dispersion stability in the constitutional layer forming material and the binding force in the constitutional layer described above. However, JP2018-200889A and WO2016/017758A1 do not describe this viewpoint.

An object of the present invention is to provide an inorganic solid electrolyte-containing composition which is excellent in dispersion stability and firmly binds solid particles in a case where a constitutional layer is formed therefrom. In addition, another object of the present invention is to provide a sheet for an all-solid state secondary battery and an all-solid state secondary battery, and manufacturing methods for a sheet for an all-solid state secondary battery and an all-solid state secondary battery, in which the above inorganic solid electrolyte-containing composition is used.

As a result of repeated studies focusing on the relationship among the solid particles, the dispersion medium, and the polymer binder in the inorganic solid electrolyte-containing composition and a constitutional layer formed from the inorganic solid electrolyte-containing composition, the inventors of the present invention obtained the following findings and further repeatedly carried out studies based on these finding, thereby completing the present invention.

That is, it was found that in a case where a polymer binder to be used in combination with an inorganic solid electrolyte is formed of a polymer into which a constitutional component having a glass transition temperature (Tg) of 50° C. or higher and an SP value of 20.0 to 25.0 MPain a case where the polymer binder is formed into a homopolymer is incorporated and is allowed to be present in a dissolved state instead of being dispersed in a particle shape in a dispersion medium present together in a composition, the relationship of the polymer binder with respect to the solid particles and the dispersion medium in the inorganic solid electrolyte-containing composition can be improved, and as a result, the reaggregation or sedimentation of the solid particles due to a lapse of time can be suppressed, whereby the excellent dispersibility immediately after preparation can be maintained. In addition, it was found that in a case where this inorganic solid electrolyte-containing composition is used as a constitutional layer forming material, the relationship (the interaction) of the polymer binder in the constitutional layer with respect to the solid particles is reinforced, whereby a constitutional layer in which the solid particles are finnly bound with firm adhesion can be realized. In addition, it was found that in a case where the inorganic solid electrolyte-containing composition containing this specific polymer binder, inorganic solid electrolyte, and dispersion medium, is used as a constitutional layer forming material, it is possible to realize a sheet for an all-solid state secondary battery, having a constitutional layer in which solid particles are firmly bonded, as well as an all-solid state secondary battery having low resistance and excellent cycle characteristics as well.

That is, the above problems have been solved by the following means.

The present invention can provide an inorganic solid electrolyte-containing composition which is excellent in dispersion stability and firmly binds solid particles in a case where a constitutional layer is formed therefrom. In addition, according to the present invention, it is possible to provide a sheet for an all-solid state secondary battery and an all-solid state secondary battery, which have a layer formed of this excellent inorganic solid electrolyte-containing composition. Further, according to the present invention, it is possible to provide manufacturing methods for a sheet for an all-solid state secondary battery and an all-solid state secondary battery, in which the above inorganic solid electrolyte-containing composition is used.

The above-described and other characteristics and advantages of the present invention will be further clarified by the following description with appropriate reference to the accompanying drawing.

In the present invention, numerical ranges indicated using “to” include numerical values before and after the “to” as the lower limit value and the upper limit value.

In the present invention, the expression of a compound (for example, in a case where a compound is represented by an expression in which “compound” is attached to the end) refers to not only the compound itself but also a salt or an ion thereof. In addition, this expression also refers to a derivative obtained by modifying a part of the compound, for example, by introducing a substituent into the compound within a range where the effect of the present invention is not impaired.

In the present invention, (meth)acryl means one or both of acryl and methacryl. The same applies to (meth)acrylate.

In the present invention, a substituent, a linking group, or the like (hereinafter, referred to as a substituent or the like), which is not specified regarding whether to be substituted or unsubstituted, may have an appropriate substituent. Accordingly, even in a case where a YYY group is simply described in the present invention, this YYY group includes not only an aspect having a substituent but also an aspect not having a substituent. The same shall be applied to a compound that is not specified in the present specification regarding whether to be substituted or unsubstituted. Examples of the preferred examples of the substituent include a substituent Z described later.

In the present invention, in a case where a plurality of substituents or the like represented by a specific reference numeral are present or a plurality of substituents or the like are simultaneously or alternatively defined, the respective substituents or the like may be the same or different from each other. In addition, unless specified otherwise, in a case where a plurality of substituents or the like are adjacent to each other, the substituents may be linked or fused to each other to form a ring.

In the present invention, the polymer means a polymer; however, it has the same meaning as a so-called polymeric compound. Further, a polymer binder (also simply referred to as a binder) means a binder constituted of a polymer and includes a polymer itself and a binder formed by containing a polymer.

[Inorganic Solid Electrolyte-Containing Composition]

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention contains an inorganic solid electrolyte having an ion conductivity of a metal belonging to Group 1 or Group 2 in the periodic table; a polymer binder containing a constitutional component satisfying (A) and (B) described later; and a dispersion medium.

This polymer binder has a characteristic (solubility) of being soluble in a dispersion medium contained in the inorganic solid electrolyte-containing composition. The polymer binder in the inorganic solid electrolyte-containing composition generally is present in a state of being dissolved in a dispersion medium in the inorganic solid electrolyte-containing composition, which depends on the content thereof. This makes it possible for the polymer binder to stably exhibit a function of dispersing solid particles in the dispersion medium and maintain the excellent dispersibility of the solid particles in the inorganic solid electrolyte-containing composition. Further, the adhesiveness between the solid particles or to the collector is reinforced, and thus it is possible to further reinforce the effect of improving the cycle characteristics of the all-solid state secondary battery.

In the present invention, the description that a polymer binder is dissolved in a dispersion medium in an inorganic solid electrolyte-containing composition is not limited to an aspect in which the entire polymer binder is dissolved in the dispersion medium, and for example, a part of the polymer binder may be present in an insoluble form in the inorganic solid electrolyte-containing composition as long as the following solubility in a dispersion medium is 80% or more.

The measuring method for solubility is as follows. That is, a specified amount of a polymer binder as a measurement target is weighed in a glass bottle, 100 g of a dispersion medium that is the same kind as the dispersion medium contained in the inorganic solid electrolyte-containing composition is added thereto, and stirring is carried out at a temperature of 25° C. on a mix rotor at a rotation speed of 80 rpm for 24 hours. After stirring for 24 hours, the obtained mixed solution is subjected to the transmittance measurement under the following conditions. This test (the transmittance measurement) is carried out by changing the amount of the binder dissolved (the above-described specified amount), and the upper limit concentration X (% by mass) at which the transmittance is 99.8% is defined as the solubility of the polymer binder in the above dispersion medium.

<Transmittance Measurement Conditions>

Dynamic Light Scattering (DLS) Measurement

Device: DLS measuring device DLS-8000 manufactured by Otsuka Electronics Co., Ltd.

Laser wavelength, output: 488 nm/100 mW Sample cell: NMR tube

In the inorganic solid electrolyte-containing composition, the polymer binder is dissolved in a dispersion medium and interacts with, preferably adsorbs to, solid particles, thereby functioning to enhance the dispersion stability of the solid particles such as the inorganic solid electrolyte. In the present invention, the adsorption of the polymer binder to the solid particles includes not only physical adsorption but also chemical adsorption (adsorption by chemical bond formation, adsorption by transfer of electrons, or the like). The polymer binder functions, in a constitutional layer formed of at least an inorganic solid electrolyte-containing composition, as a binder that causes solid particles of an inorganic solid electrolyte (as well as a co-existable active material, conductive auxiliary agent, and the like) or the like to mutually binds therebetween (for example, between solid particles of an inorganic solid electrolyte, between solid particles of an inorganic solid electrolyte and an active material, or between solid particles of an active material). Further, it also functions as a binding agent that firmly binds the base material such as a collector and the solid particles.

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention is preferably a slurry in which the inorganic solid electrolyte is dispersed in a dispersion medium.

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention has excellent dispersion stability and can firmly bind solid particles to other solid particles and furthermore to a base material in a case where a constitutional layer is formed therefrom. In a case where this inorganic solid electrolyte-containing composition is used as a constitutional layer forming material, it is possible to realize a sheet for an all-solid state secondary battery, which has a low-resistance constitutional layer in which solid particles firmly adhere, and furthermore, an all-solid state secondary battery which has a high conductivity (low resistance) and excellent cycle characteristics as well.

Although the details of the reason for the above are not yet clear, they are conceived to be as follows. That is, it is presumed to be because a polymer binder which is formed of a polymer into which a constitutional component having a glass transition temperature (Tg) of 50° C. or higher and an SP value of 20.0 to 25.0 MPain a case where the constitutional component is formed into a homopolymer is incorporated and dissolved in a dispersion medium present together in a composition can improve or modify the relationship with respect to the solid particles and the dispersion medium in the inorganic solid electrolyte-containing composition and the constitutional layer.

Specifically, the polymer binder having the above-described configuration is uniformly present in the inorganic solid electrolyte-containing composition in a state of being dissolved in a dispersion medium with the molecular chain thereof being spread. In addition, the above-described constitutional component in the polymer that constitutes the polymer binder causes the polymer binder to exhibit high wettability to the solid particles based on a specific SP value. Therefore, in the inorganic solid electrolyte-containing composition, the initial dispersibility of the solid particles (the dispersibility at the time of preparing the composition) is improved, and furthermore, the reaggregation or precipitation of the solid particles due to a lapse of time is suppressed, whereby excellent initial dispersibility can be maintained. Such an inorganic solid electrolyte-containing composition having excellent dispersion stability makes it possible to form a constitutional layer having excellent surface properties having less coating unevenness (non-uniformity of a layer thickness, uneven distribution of solid particles, and the like) and realize a high conductivity.

On the other hand, in the process of forming a film of the inorganic solid electrolyte-containing composition, since the polymer contains the above-described constitutional component, the polymer binder is solidified to a high strength (high hardness) based on a specific glass transition temperature (Tg). Therefore, the interaction between the polymer binder and the solid particles in the constitutional layer to be formed into a film is enhanced, and the binding property between the solid particles and furthermore, the binding force between the solid particles and the base material can be reinforced. The constitutional layer in which the binding force of the solid particles is reinforced is less likely to generate voids even in a case where the all-solid state secondary battery is repeatedly charged and discharged, cycle characteristics can be improved, and furthermore, the tolerance (the suppression of occurrence of defects and the suppression of peeling from a base material) to a stress that acts during the manufacturing process can be also improved, which makes it also possible to suppress an increase in resistance (a decrease in conductivity).

In the present invention, as described above, the interaction (the relationship) among the inorganic solid electrolyte, the dispersion medium, and the polymer binder in the inorganic solid electrolyte-containing composition and the constitutional layer is improved, and it is possible to realize the excellent dispersion stability of the inorganic solid electrolyte-containing composition and the firm binding of solid particles in a case where a constitutional layer is formed therefrom. Therefore, it is possible to apply industrial manufacturing, for example, a roll-to-roll method having high productivity as manufacturing methods for a sheet for an all-solid state secondary battery and an all-solid state secondary battery using the inorganic solid electrolyte-containing composition according to the embodiment of the present invention. In addition, in a case of using the inorganic solid electrolyte-containing composition according to the embodiment of the present invention, it is possible to realize a sheet for an all-solid state secondary battery, including a constitutional layer in which solid particles firmly adhere to each other while exhibiting a sufficiently low resistance due to having excellent surface properties with little coating unevenness, and furthermore, an all-solid state secondary battery having both low resistance (high conductivity) and excellent cycle characteristics. Furthermore, in a case of using the sheet for an all-solid state secondary battery, produced using the inorganic solid electrolyte-containing composition according to the embodiment of the present invention, as an electrode (a laminate of a collector and an active material layer) of an all-solid state secondary battery, it is possible to further improve the cycle characteristics while maintaining low resistance.

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention is preferably used as a material (a constitutional layer forming material) for forming a solid electrolyte layer or an active material layer, where the material is for a sheet for an all-solid state secondary battery (including an electrode sheet for an all-solid state secondary battery) or an all-solid state secondary battery. In particular, it can be preferably used as a material for forming a negative electrode sheet for an all-solid state secondary battery or a material for forming a negative electrode active material layer, which contains a negative electrode active material having a large expansion and contraction due to charging and discharging, and high cycle characteristics and furthermore, high conductivity can be achieved in this aspect as well.

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention is preferably a non-aqueous composition. In the present invention, the non-aqueous composition includes not only an aspect including no moisture but also an aspect where the moisture content (also referred to as the “water content”) is preferably 500 ppm or less. In the non-aqueous composition, the moisture content is more preferably 200 ppm or less, still more preferably 100 ppm or less, and particularly preferably 50 ppm or less. In a case where the inorganic solid electrolyte-containing composition is a non-aqueous composition, it is possible to suppress the deterioration of the inorganic solid electrolyte. The water content refers to the water amount (the mass proportion to the inorganic solid electrolyte-containing composition) in the inorganic solid electrolyte-containing composition, and specifically, it is a value measured by carrying out filtration through a 0.02 μm membrane filter and then Karl Fischer titration.

The inorganic solid electrolyte-containing composition according to the aspect of the present invention includes an aspect containing not only an inorganic solid electrolyte but also an active material, as well as a conductive auxiliary agent or the like (the composition in this aspect may be referred to as the “electrode composition”).

Hereinafter, components that are contained and components that can be contained in the inorganic solid electrolyte-containing composition according to the embodiment of the present invention will be described.

<Inorganic Solid Electrolyte>

The inorganic solid electrolyte-containing composition according to the embodiment of the present invention contains an inorganic solid electrolyte.

In the present invention, the inorganic solid electrolyte is an inorganic solid electrolyte, and the solid electrolyte refers to a solid-form electrolyte capable of migrating ions therein. The inorganic solid electrolyte is clearly distinguished from the organic solid electrolyte (the polymeric electrolyte such as polyethylene oxide (PEO) or the organic electrolyte salt such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)) since it does not include any organic substance as a principal ion-conductive material. In addition, the inorganic solid electrolyte is solid in a steady state and thus, typically, is not dissociated or liberated into cations and anions. Due to this fact, the inorganic solid electrolyte is also clearly distinguished from inorganic electrolyte salts of which cations and anions are dissociated or liberated in electrolytic solutions or polymers (LiPF, LiBF, lithium bis(fluorosulfonyl)imide (LiFSI), LiCl, and the like). The inorganic solid electrolyte is not particularly limited as long as it has an ion conductivity of a metal belonging to Group 1 or Group 2 in the periodic table and generally does not have electron conductivity. In a case where the all-solid state secondary battery according to the embodiment of the present invention is a lithium ion battery, the inorganic solid electrolyte preferably has a lithium ion conductivity.

As the inorganic solid electrolyte, a solid electrolyte material that is typically used for an all-solid state secondary battery can be appropriately selected and used. Examples of the inorganic solid electrolyte include (i) a sulfide-based inorganic solid electrolyte, (ii) an oxide-based inorganic solid electrolyte, (iii) a halide-based inorganic solid electrolyte, and (iv) a hydride-based inorganic solid electrolyte. The sulfide-based inorganic solid electrolytes are preferably used from the viewpoint that it is possible to form a more favorable interface between the active material and the inorganic solid electrolyte.

(i) Sulfide-Based Inorganic Solid Electrolyte

The sulfide-based inorganic solid electrolyte is preferably an electrolyte that contains a sulfur atom, has an ion conductivity of a metal belonging to Group 1 or Group 2 in the periodic table, and has electron-insulating properties. The sulfide-based inorganic solid electrolytes is preferably a sulfide-based inorganic solid electrolyte which contains, as elements, at least Li, S, and P and have a lithium ion conductivity; however, it may appropriately contains elements other than Li, S, and P.